The long-term storage of liquid parenteral drugs in pre-filled syringes provides simplicity and convenience for patients and clinicians. The use of such syringes enables skipping the preparatory stage of extracting a drug from a liquid vial (or reconstituting from powder) and then filling into a syringe for injection. An even easier modality of delivering drugs is to use a patch-pump or patch-injector rather than a syringe. The use of such devices reduces “needle shock” and can also enable slow-infusion of drugs in place of rapid injection and facilitate delivery of large-doses of drugs, having a range of viscosities.
The long-term storage of drugs in syringes or other liquid containers requires that those containers be made from a very limited variety of materials, all of which have to be shown to cause minimal damage to the drug contained within even during direct and prolonged contact with the drug. Historically, this material has always been glass, but in more recent years glass syringes have been shown to have some problems and incompatibilities with biological. In particular, numerous drugs—especially biologics—can be adversely affected by the release of soluble alkali from the glass containers. Moreover, there is a known problem of Bovine serum albumin (also known as BSA protein) absorption on the container surface. Additionally, the silicone which is used as a lubricant in a syringe and the tungsten residue from the pin that is used to form the nozzle of the syringe into which the needle is embedded can also adversely affect these drugs. As a partial solution to these problems, some pre-filled syringes are starting to be fabricated from certain inert plastics, which exhibit low protein absorption and high compatibility with biological drugs. To date, however, there is no such appropriate solution for long term drug containers for application in a patch-pump or patch-injector housing.
Patch-pumps or patch-injectors or micro-infusers, or other such devices that transfer drug from an external source onto or through the skin, are drug-delivery devices which ideally have a very slim profile and are relatively flat, so as to provide maximum comfort to the patient. Syringe-type drug reservoirs represent a very inefficient design of a drug reservoir in terms of minimizing the size of the patch-pump device. This is due to the fact that (a) the rigid cylindrical design of syringes require that the reservoir portion of the patch pump needs to have at least the thickness dictated by the diameter of the syringe plus the thickness of the walls of the housing, and (b) the initial length of the syringe reservoir needs to be at least the length of the portion of the syringe which holds the drug, plus the protruding length of the plunger.
One approach to integrating a relatively flat drug reservoir into a patch-pump is described in U.S. Pat. No. 7,250,037 in which the drug reservoir has a domed shape. This dome is bounded on one side by a multi-layer film and from the other by a “dished-out” section of a rigid plastic. A spring presses against the multi-layer film in order to dispense the drug. As this spring extends, said multi-layer film is pressed into the “dished-out” section in the rigid plastic, thereby expelling the drug. While this configuration does enable the actuator (in this case a spring) to expel the contents of the reservoir, in terms of space-efficiency it is non-optimal. The optimal exploitation of the volume would of course be the use of 100% of the volume of a cylinder defined by the depth of the reservoir (h) multiplied by its area (πr2). For example, assuming that the dome is semi-spherical, with a radius of 15 mm, then the volume enclosed using this approach would be ⅔πr3 which is 9.3 mL; whereas the volume of a cylinder with this radius and equivalent depth (15 mm) would be 14.6 mL. Thus it is clearly more space-efficient to have a drug-reservoir design which more closely approximates a cylinder.